The present invention relates to a method and apparatus for detecting changes in acceleration of a moving object and, more particularly, to such a method and apparatus useful in a wheel slip correction scheme on traction vehicles.
In many applications there is a need to sense a change in acceleration of a moving object. For example, in vehicle drive systems a change in rotational acceleration of a wheel may be indicative of a loss of adhesion between the wheel and a tractive surface along which the wheel is rolling. This change in acceleration may occur either when the wheel is being driven to increase velocity of the vehicle or when the wheel is being braked to thereby decrease vehicle velocity. Braking of a wheel is sometimes referred to as deceleration. For purposes of illustrating the advantages of the present invention, the invention will be described in conjunction with a wheeled vehicle and, more particularly, in conjunction with a control system for an electric traction motor drive system. However, it will be apparent to those skilled in the art that the present invention is adaptable to many other applications.
Traction vehicles, such as locomotives or transit cars, commonly utilize a plurality of individually powered axles. Loss of adhesion, or otherwise stated, a decrease in the coefficient of friction between wheel and rail below that required to maintain a powered wheel in rolling contact with the rail, will result in slipping of the wheel on the rail. Slipping is generally defined as a condition wherein the rotational velocity of the wheel is greater than that velocity required to drive the vehicle at its actual velocity. When one, or all, of the powered wheels of a vehicle slips, there may not be any noticeable loss of tractive efffort to warn the vehicle operator of such slippage until the slip is severe. Uncorrected wheel slips cause loss of vehicle performance and may cause damage to propulsion equipment, vehicle wheels and the rail on which the equipment is operating. Therefore, some automatic means of detecting and correcting wheel slips is highly desirable.
A related problem to wheel slip is wheel slide, which may occur during electrical or mechanical braking of an electrically powered vehicle when the applied braking torque becomes excessive for the degree of adhesion between the wheel and rail. Sliding is generally defined as a condition wherein the rotational velocity of the wheel is less than that velocity corresponding to the actual velocity of the vehicle. Slipping and sliding are similar in that both may occur as a result of excessive torque of the traction motor. Sliding wheels will develop flat spots whereas slipping wheels will tend to become undersized. Persistent recurrence of either slipping or sliding is highly undesirable.
Two types of wheel slipping or sliding are common. In a first type the wheels of one of the individually powered axles of a vehicle may slip or slide. In a second type the wheels of all of the axles may simultaneously slip or slide. The latter type of condition is referred to as a synchronous slip or slide condition. Slipping or sliding of a single axle may be detected by comparing the speed of the individual axle with the speed of other axles. A synchronous slip condition, however is not susceptible to such a method since all the axles will be revolving at the same velocity. One method for detecting a synchronous slip condition is to set a maximum or overspeed limit and a minimum or underspeed limit for the wheels of the electrically propelled vehicle and then to detect when the wheel speed is beyond the limits imposed by this control system. Such a control system is described in U.S. Pat. No. 3,210,630 -- Zelina issued Oct. 5, 1965, and assigned to the General Electric Company. One of the difficulties with a system of this nature is, however, that the damage may have already occurred to either the control system, the vehicle or the rails before the system indicates that an overspeed or underspeed condition exists. Furthermore, a slipping or sliding wheel actually has a lower coefficient of friction than a rolling wheel and thus a net loss of tractive effort will occur. A slip-slide detection system would ideally detect the slippage prior to an excessive loss of adhesion.
In propulsion systems in which the driving elements are alternating current (a-c) motors, synchronous slip is generally the only type of slip which the system will experience. In a typical application a separate a-c traction motor is geared to each axle of a vehicle, and the stator windings of all the traction motors on the vehicle are connected in parallel for energization from a single power source such as, for example, an inverter supplying stator excitation of controllable amplitude and frequency. The speed of the a-c motors is a function of the frequency of the a-c power supplied by the inverter. Since all of the motors are excited at the same frequency, the only difference in speed will be that imposed by the slight differential in motor slip between the respective driving motors. The motor slip frequency is normally in the range of one to five cycles which is in the vicinity of 0.5 to 1.5% of maximum speed. The difference in velocity between a non-slipping wheel and a slipping wheel will therefore be negligible. What happens when the wheels on one axle begin to slip or slide is that the associated traction motor tends to unload and the decrement is picked up by the other traction motors which consequently impose increased torque on the non-slipping wheels until they too lose adhesion and begin to slip or slide. Accordingly, in an a-c traction motor drive system, comparison of relative speeds between individually powered axles does not represent an effective method for determining a wheel slip or slide condition.
In some applications the electrically powered vehicle may have only a single drive train. For example, a geared drive to all axles may be used. In a single drive train system comparison of axle speeds is not possible. Most single drive train slip/slide detection systems utilize either an upper and lower wheel speed limit or an upper and lower motor current limit as a condition for detection wheel slip or slide. This type of detection system requires that a limit condition occur before the system can be responsive to the wheel slip or slide. Thus, the damage which can be done by wheel slip or slide may have already occurred before the detection system can react and provide a correction.
Accordingly, it is an object of the present invention to provide a method and apparatus for detecting a change in acceleration of an object without requiring a comparison of the acceleration of that object with acceleration of another object.
It is a further object of the present invention to provide an improved method and apparatus for detecting a wheel slip or a wheel slide condition in a traction vehicle.
It is a still further object of the present invention to provide an improved method and apparatus for detecting a synchronous wheel slip or wheel slide condition in a traction vehicle.